harmonic mean of 1 and 2 is 2/(1 + 1/2) = 4/3. Such an integer is called Ore
harmonic number or harmonic divisor number. According the theorem of Ore;
every perfect number is harmonic.
n is called superperfect number if σ(σ(n)) = 2n and every even superperfect
number n must be a power of 2, that is, 2p−1 such that 2p − 1 is a Mersenne
prime.